This light dimmer has been specially designed for use with fluorescent lamps (FLs). The circuit contains only a few components and can be built on a circuit board of only 2 by 3 cm if you use SMD components. The mains voltage is rectified by D1 to D4 and this waveform goes to the in-series connected fluorescent lamp and a thyristor.
During the day the thyristor will not receive any gate current so the lamp will remain off. At night the thyristor will receive a continuous gate current so that the fluorescent lamp stays fully on. The light/dark detection circuit is built around T1 to T4. This part is fed directly from the rectified mains voltage via R1/D5/C1.
Phototransistor T1 measures the amount of ambient light. During the day, when there is sufficient light, T1 conducts. In that case, T2 and T3 will block and T4 conducts so that the thyristor does not receive any current. When dusk falls the voltage across electrolytic capacitor C2 increases. At some point, this will become high enough so that T2 and T3 will conduct.
T4 will not receive any base current anymore and blocks, so the thyristor will receive continuous gate current via voltage divider R6/R7/R8, and the lamp will light up. R9 and R10 provide for some hysteresis in the switching behavior of T2 and T3, so that the circuit does not repeatedly turn on and off when dusk falls. When building the circuit make sure that it is electrically safe since it is directly connected to the mains voltage.